You'll need very deep depth of field, which should be easy to obtain with a G9 since it is a small sensor compact and those cameras inherently have lots of DOF. Use a solid, sturdy tripod, lowest possible ISO and DO use flash, but use it properly, off camera and remotely triggered. That's how the to get the crystal clear focus, saturated colors and dynamically lit tank photos.
You will need to use one or two remote flashes. Use remote slave triggers on the flash guns and run them in manual mode. Auto modes are gonna get fooled every time here. Mount the flashes over the tank using a flash stand ( a boom mic stand and duct tape also work), firing down into the water, and use a polarizing filter on the camera lens to help reduce any reflection from ambient lights. The flashes will be your main light so make sure you set a custom white balance to get your colors correct and properly saturated. Standard flash modes won't cut it because firing through all that water is going to bias the light towards blue, so you need to set the custom white balance. The use of flash will also allow you to use a much shorter shutter speed. Most of the blur in photos is not focus issues but usually movement by the fish....the flash will freeze the movement and when properly balanced, will light both the fish and tank in sharp focus with balanced colors.
Stop the lens down to around F5.6 or so. As for focus, the best way would be to manually focus your camera on something in the front third of your tank. Set your shutter speed to your highest flash sync speed and take some test shots. Adjust your flash outputs as necessary to get the most pleasing results. Then write everything down!
It's not as easy as point and shoot but it is not beyond the capabilities of a G9. A DSLR would make things easier, but it can be done if you work at it.
